1. Field of the Invention
The present invention relates to a block copolymer of polyesters and polycarbonates and a process for their preparation. More particularly, the present invention relates to biodegradable and hydrolyzable thermoplastic resin. The copolymer of the invention is decomposable by micro-organisms living in the soil or in the water, so that it can be used as non-polluting and clean plastic material. This quality gives the polymer a broad scope of uses, functionally as varied as a material for a fishing line, a fishing net, a sheet for agricultural use, a packing film, a surface film for paper packing, a material for lamination layers, a plasticizer, a compatibilizer etc.
Also, the polymer according to the invention is biocompatible, so that it can be hydrolyzed and absorbed by a living body. It may therefore be used in medical materials e.g. for surgical sutures or clips without requiring the extraction of stitches.
2. Prior Art
In recent years, the environmental pollution caused by the disposal of non-degradable or difficultly degradable films or fishing lines has become an important issue. To solve this problem, attempts have been made to find a plastic material that is easily degradable by the micro-organisms living in nature. Also, there is currently research into a bio-compatible plastic material that can be degraded and absorbed by living bodies. In this case, the material preferably has to maintain its strength only during the regeneration of tissue and to disappear rapidly thereafter.
Aliphatic polyesters are known to be biodegradable and bio-compatible. Among them, poly (R)-3-hydroxybutyric acid (hereinafter referred to as P[(R)-3HB]) accumulated in microbial cells is drawing particular attention because it is a thermoplastic resin having a high melting temperature, e.g. around 180.degree. C. (cf. "Biodegradable Macromolecular Materials", edited by Yoshiharu DOI, p21, 1990, published by Kogyo Chosa Kai). There exist many known microorganisms that accumulate various kinds of poly (R)-3-hydroxyalkanoic acid in their cells (page 26 of the aforementioned document).
A copolymer of (R)-3-hydroxybutyric acid and (R)-3-hydroxyvaleric acid that shows improved properties over the polymer P[(R)-3HB] was reported by P. A. HOLMES (Phys. Technol., 1985 (16), p32). Also, a copolymer of (R)-3-hydroxybutyric acid and 4-hydroxybutyric acid was reported by Y. DOI et al. (Polym. Commun., 1988 (29), 174). These copolymers are formed as random copolymers (Macromolecules 1986 (19), 2860; Macromolecules, 1988 (21), 2722).
Recently, various random copolymers of polyesters of bio-degradable high molecules containing (R)-3-hydroxybutyric acid units were chemically synthesized in the presence of a distannoxane catalyst: the copolymerisation was effected by ring-opening of (R)-.beta.-butyrolactone (hereinafter referred to as (R)-BL) and of various other lactones (Macromolecules, 1993 (26), 4388).
Meanwhile, random copolymers of glycolides and lactides have already been used for bio-absorbable surgical sutures ("Bio-compatible materials--their functions and applications", 1993, p127, edited by Yoshito IKADA, published by Nippon Kikaku Kyokai).
The random copolymers of polyesters synthesized as described above decrease their crystallinity and reduce their fragility. On the other hand, the random copolymerisation usually lowers the melting temperature. Therefore, to obtain a biodegradable aliphatic polyester having a melting temperature at least as high as 100.degree. C., the polymer has to contain a large proportion of optically active units of 3-hydroxybutyric acid or of L-lactic acid, or glycolic acid units. This in turn poses a serious industrial problem due to increased production costs. Further, it is a rather difficult task to obtain a soft film or rubber from the random copolymers while simultaneously maintaining a high melting temperature and strength.
To solve the problem, block copolymers of polyesters or block copolymers of polyesters and polycarbonates may preferably be synthesized. According to M. S. Reeve et al. (Macromolecules, 1993 (26), 888), a microbially produced polymer P[(R)-3HB] was submitted to a methanolysis to obtain oligomers. Terminal hydroxyl groups of the obtained oligomers were then reacted with triethylaluminium, thereby forming polymerisation starters. Subsequently, the starters were reacted with .epsilon.-caprolactone (referred to as CL), L-lactide (L-LA) or D,L-lactide (DL-LA) to obtain a block copolymer of polyesters P[(R)-3HB] and poly .epsilon.-caprolactone, referred to as P[(R)-3HB]-PCL, a block copolymer of polyesters P[(R)-3HB] and poly L-lactide, referred to as P[(R)-3HB]-P(L-LA) and a block copolymer of polyesters P[(R)-3HB] and poly D,L-lactide, referred to as P[(R)-3HB]-P(DL-LA). However, in this method, the microbially produced polymer P[(R)-3HB] is used as a starting material, thereby incurring a higher production cost.
On the other hand, block copolymers of polyesters and polycarbonates were reported in U.S. Pat. No. 4,243,775 and international application WO 89/05664. These are block copolymers of lactones and carbonates obtained by using alcohols as starters in the presence of tin-type catalyst.
Further block copolymers of lactones and carbonates were also obtained by using polyethyleneglycols as starters in the presence of tin-type catalyst and reported in U.S. Pat. No. 4,857,602. Yet further block copolymers of lactones and carbonates were obtained by using butyl lithium as catalyst and reported in DE 3 607 627 A1. Further, other lactone-carbonate type block copolymers were disclosed in EP 0 427 185 A2.
To copolymerize tactones and cyclic carbonates in block, it is necessary to rely upon a living polymerization: first lactones are ring-opened and polymerized in the presence of a catalyst; then the terminals of the polymer formed are bound to the catalyst and kept in an active state. Subsequently, when cyclic carbonates are added, these are inserted successively between the polymer terminals and the catalyst, thereby extending the block copolymer chain of polyesters and polycarbonates.
However, in the system of catalysts reported in the aforementioned documents, high molecular block copolymers of polyesters and polycarbonates could not be obtained between lactones, especially (R)-BL or (S)-BL, and cyclic carbonates.